Some types of sensors such as Hall sensors and cam shaft sensors require a magnet for proper operation. These sensors are typically manufactured as a semiconductor die that is attached to a leadframe and embedded in a molding compound to form a sensor package.
In some cases, the magnet is overmolded to the sensor package. Overmolding is a technique that allows for a previously molded part to be reinserted in a mold to form a new plastic layer around the first part. Overmolding a magnet to a plastic sensor package typically involves melting pellets of magnetized material and injecting the melted material into a mold that includes the plastic package. The magnet is realized from the cured molding compound. That is, there is no physically separate magnet. Instead, the magnet is formed from the overmolded plastic itself. Overmolding requires a relatively high Tg (glass transition temperature) molding compound e.g. Tg>180° C., resulting in significant thermal stress on the sensor package. Also, molding compounds with Tg>180° C. such as multi-functional thermoplastics have a high risk of delamination when use as an overmold. Furthermore, the magnetic material must tolerate high operating temperatures (>300° C.) associated with overmolding, limiting the types of materials that can be used.
In other cases, the magnet is a separate component attached to the exterior surface of the molded sensor package e.g. by an adhesive. Such a sensor package has low electrical accuracy due to imprecise placement of the magnet, requiring the sensor die to have an otherwise large tolerance. Also, the overall package cost increases because of the production higher cost associated with serial processing a single magnet plus a pole piece. Furthermore, the magnet size and magnetic field is limited with this approach.